Mammalia: Visceral Systems 663 



Fig. 500. Diagram of male genitalia of beaver, Casior canadensis. Penis re- 

 tracted into the floor of the shallow cloaca, (a) Anns; (ag) anal gland; (6) urinary 

 bladder; (gv) gland of vas deferens; {oa) opening of anal gland; (op) os penis; 

 (p) prostate; (pp) preputial gland; (r) rectum; (u) urethra; (vd) vas deferens. 

 (After Weber. Courtesy, Kingsley: "Comparative Anatomy of Vertebrates," 

 Philadelphia, The Blakiston Company.) 



penis is directed downward or more or less forward (Fig. 496). In 

 those insectivores and rodents which have more or less of an ectodermal 

 cloaca as in marsupials, the retracted penis is drawn into the cloaca, 

 pointing backward. The right and left scrotal sacs lie laterally in the 

 perineal region but are so shallow as to cause little external protrusion. 



Accessory Genital and Anal Glands 



At certain regions in the more posterior part of the male urino- 

 genital tract, the walls of the passages are glandular or may give rise 

 to appended glandular organs. Their common function seems to be 

 secretion of a mucous fluid which serves as a carrier for the sperma- 

 tozoa and provides a medium favorable for their survival and activity. 

 In some cases (rodents) the secretion coagulates in the vagina, thus 

 favoring fertilization by preventing escape of the spermatic fluid. 



These glands are distinguished mainly by their position in relation 

 to the passages (Fig. 492). In the more posterior region of each vas 

 deferens, but anterior to its ejaculatory region, the wall may be thick 

 and glandular or, more commonly, it gives rise to a pair of large and 

 more or less complexly subdivided glands, often called "seminal 

 vesicles," but better known as vesicular glands because they are 

 essentially secretory rather than for mere storage of sperm. Especially 

 important glands occur along the neck of the bladder and the urethra. 

 The prostate is usually a massive gland divided into a varying number 

 of lobes, situated just posterior to the junction of the vasa deferentia 

 w it li the neck of the bladder. At the base of the prostate, or embedded 

 in the gland, is commonly found a small and usually bilobed vesicle 

 protruding from the wall of the urethra. This prostatic vesicle or 

 utriculus is a remnant of the embryonic Miillerian ducts. It is usually 

 known as the "uterus masculinus," but, being derived from the 



